This invention relates to a release liner for pressure sensitive adhesive tapes.
Release coatings are well known in the pressure sensitive adhesive tape industry to provide a release surface so that a tape can be unwound from a roll without the adhesive sticking to the backside of the tape. The release coating can also be coated onto a liner which serves as a carrier for a pressure sensitive adhesive transfer tape or a double coated tape, both of which are tacky on both sides of the tape.
Materials commonly used for release surfaces and coatings are silicone compositions because they can be formulated to provide varying levels of release from a xe2x80x9cpremiumxe2x80x9d release (i.e., the force to unwind the tape is very low) to a xe2x80x9clow adhesion backsizexe2x80x9d release which typically requires a greater force to unwind.
The primary deficiency of silicone release coatings is the potential contamination that may be associated with them. When workers handle a silicone release coated tape or liner, silicone can transfer to their hands, work gloves, or clothing. The silicone can be further transferred from their hands to surfaces to be painted, and cause problems such as poor adhesion of the paint to the contaminated surface, and nonwetting of the paint in areas where the silicone is present on the surface. Silicone release materials also tend to be costly, and silicone coated materials may be difficult to recycle.
Polyethylene has also been used as a release material and can be coated onto Kraft papers for a release liner or it can be used as a single layer film, or a multi-layer film with polyethylene co-extruded with or laminated to a base layer such as high density polyethylene, and the like. Higher density polyethylenes are limited in use to certain adhesives and are found to be unsuitable for very aggressive pressure sensitive adhesives because the adhesion between the polyethylene and the adhesive is so high that it can be difficult to start removing the liner. Since the release force is high, the adhesive and/or the liner can also be damaged as one tries to separate the adhesive from the liner.
There remains an ongoing need for controlled release liners for pressure sensitive adhesives that do not have a silicone release layer.
The present invention provides a novel sheet material comprising i) a pressure-sensitive adhesive film comprising an acrylate-based pressure-sensitive adhesive polymer; and ii) a release film comprising a first layer comprising polyolefin polymer having a density of no greater than about 0.90 g/cc and a CDBI of greater than about 70 percent, the pressure-sensitive adhesive film being in contact with the first layer of the release film.
In a preferred embodiment there is provided a sheet material which is substantially free of any silicone release coating.
In another preferred embodiment the release film comprises a second layer which is preferably a polybutylene.
In the practice of the invention, the materials useful for the first layer of the release film are copolymers of ethylene and alpha-olefins having from 3 to about 10 carbon atoms. Suitable alpha-olefins include butene-1, hexene-1, octene-1, and combinations thereof. Copolymers of ethylene and butene-1 are preferred for use with acrylate-based pressure sensitive adhesives.
The copolymers are generically described as very low density polyethylene (VLDPE) and have been described as xe2x80x9cplastomersxe2x80x9d, a polymer having thermoplastic and elastomeric characteristics. Preferred copolymers have a density no greater than 0.90 grams per cubic centimeter (g/cc). More preferred are copolymers having a density of no greater than 0.89 g/cc and most preferred are those having a density no greater than 0.88 g/cc. Lower density copolymers are found to provide a lower release value, and values can be changed by blending copolymers of varying types.
The useful copolymers preferably have a narrow molecular weight distribution as defined by having a polydispersity of between about 1 and 4, and more preferably between about 1.5 and 3.5. Polydispersity is defined as the ratio of the weight average molecular weight to the number average molecular weight.
The useful copolymers can be defined by a parameter which has been characterized as the xe2x80x9ccomposition distribution breadth indexxe2x80x9d (referred to hereinafter including the claims as xe2x80x9cCDBIxe2x80x9d). The CDBI is defined as the weight percent of the copolymer molecule, having a comonomer content within 50 percent (i.e., xc2x150%) of the median total molar comonomer content. The CDBI and the method for its determination is described in U.S. Pat. No. 5,206,075, incorporated herein by reference. The CDBI of suitable copolymers for use in this invention is preferably greater than about 70 percent, more preferably is greater than about 80 percent, and most preferably is greater than about 90 percent.
Suitable copolymers are commercially available from Exxon under the EXACT tradename. These copolymers form films that are not fusible at ambient temperature, and will not block, i.e., stick to underlying layers, when the film is wound into a roll.
In a preferred embodiment of the first layer of the release liner consists essentially of the copolymer having a density of no greater than 0.90 g/cc, and is substantially free of any polyethylene having a density of 0.91 g/cc or greater.
As used herein, the terms release liner, liner, and release films will be used interchangeably. The release film can be prepared by extruding the copolymers as a single self supporting film where there is no second layer present. The film thickness may range from about 0.1 millimeter to about 0.4 millimeters, although the thickness will generally depend upon considerations such as the strength needed in the release liner, ease of handling the film, flexibility needed for the liner, and the like.
The release film may also be prepared by extruding the copolymer onto a base material, or by co-extruding the copolymer with one or more other polymers to form multi-layer films to provide additional strength and/or stiffness. Examples of suitable base materials include cellulosic materials such as paper, woven nonwoven fabrics, films such as nylon, polyester, polyolefins, acrylonitrile butadiene styrene, and sheet materials made of materials such as metal, ceramic, or plastic. The copolymer film can also be attached to the base material using an appropriate laminating adhesive. The base material can also be a sheet material suitable for thermoforming or vacuum forming so that trays can be formed having a release surface for holding pressure sensitive adhesive coated parts.
The thickness of the copolymer film on a base layer should be sufficient to provide the desired release force, and may be less than for a self-supporting sheet.
The films are typically extruded onto chill rolls having various surfaces such as a matte finish or a smooth finish to provide matte or glossy surfaces on the release liner. Chill rolls can be treated or coated with materials to prevent sticking of the extrudate to the roll surface, such as Teflon(trademark) or a plasma coating. Silicone rubber rolls may also be used for this purpose. The extruded films can also be extruded between a chill roll and a smooth polyester film to provide a glossy surface on one side and a matte surface on the other side, or between two polyester liners for two glossy surfaces.
Multi-layer films may be prepared by known co-extrusion processes. Other polymers that can be co-extruded with the copolymer include polyolefins such as polypropylene, polyethylene, polybutylene, and mixtures thereof, and polyesters. Co-extrusion with polyolefins is especially useful for making recyclable or re-usable liners.
In a multi-layer film construction, the polymers and co-polymers can be selected to exhibit differential release, i.e., the pressure sensitive adhesive will release from one side with a noticeably lower force than from the other side.
Preferred polymers for co-extrusion with the copolymer of the first layer is polybutylene homopolymers and copolymers. Polybutylene homopolymers are especially preferred in the second layer of the release film because they provide strength to the film and also serves as a high differential release liner. A useful homopolymer for employment in the base layer or second layer of the release film is polybutylene. Useful polybutylene hompolymers and copolymers are available from Shell Chemical Co. under the DURAFLEX tradename. Acrylate-based pressure sensitive adhesives were observed to have a higher release force from the polybutylene surface than from the polyethylene copolymer surface.
In some applications it is desirable to have a liner width slightly greater than the width of the pressure sensitive adhesive. This is especially true of thicker pressure sensitive adhesives, i.e. greater than about 0.5 millimeters which may flow slightly when wound in a roll and the greater width helps to prevent adhesive from flowing out onto the adjacent wrap of tape and causing blocking of the tape.
For these applications, it is preferred to use a base material or co-extruded material that will form a film which when stressed for a short time, i.e., less than about a second, the film will recover less than 100% of its stretch. For example, if a film having a length of 1.0 cm is stretched to a length of 2.0 cm and released, the film will recover to a length greater than 1.0 cm.
In a typical process to make pressure sensitive adhesive transfer tapes, an adhesive composition is coated onto a release liner. The adhesive is then cured to form a gelled film on the liner, and the release liner with the adhesive is rolled up into a large roll. Alternatively, the adhesive may be coated and cured on one liner, and then transferred onto a different liner before converting. The adhesive coated sheet is then converted into narrow rolls by slitting the large roll and winding the narrow width tape onto cores for customer use.
During converting of thick sheets, i.e., greater than about 0.5 mm, the slitting knife or razor blade penetrates the sheet material from the backing side. As the knife penetrates the liner and adhesive, the width of the knife exerts sufficient forced to stretch both the liner and adhesive. After slitting, the adhesive being elastic in nature, shrinks back to the slit width, while the liner recovers less than 100% so that it is slightly wider than the adhesive.
It has been found that polybutylene homopolymers and polybutylene copolymers comprising a minor amount (i.e., no more than five percent) of a comonomer such as ethylene, in the second layer of the release film are particularly preferred to provide a film having low stress recovery. Without wishing to be bound by theory, the polybutylene apparently strain crystallizes when it is stressed or stretched during slitting and the crystallization prevents recovery of the stretch. The liners of the invention are useful as release films for acrylate-based pressure sensitive adhesives. Such adhesives include homopolymers and copolymers of monofunctional unsaturated acrylic or methacrylic acid ester monomers of non-tertiary alcohols having from about 1 to 20 carbon atoms, and preferably from 4 to 12 carbon atoms. A comonomer may optionally by included to improve the cohesive strength of the adhesive. Comonomers useful in making the copolymers typically have a higher homopolymer glass transition temperature than the glass transition temperature of the acrylic acid ester homopolymer.
Suitable acrylic acid ester monomers include 2-ethylhexyl acrylate, isooctyl acrylate, isononyl acrylate, n-butyl acrylate, decyl acrylate, dodecyl acrylate, and mixtures thereof. Preferred monomers include isooctyl acrylate, n-butyl acrylate, and mixtures thereof.
Useful reinforcing comonomers include acrylic acid, methacrylic acid, itaconic acid, acrylamide, substituted acrylamides, N-vinyl pyrrolidone. N-vinyl caprolactam, isobornyl acrylate, and cyclohexyl acrylate. Preferred comonomers include N-vinyl caprolactam, substituted acrylamides such as N,N-dimethylacrylamide, and isobutyl acrylate.
Suitable initiators are used to make pressure adhesives in the practice of the invention. The types and amounts initiators are used in amounts suitable to effect the type of polymerization used, e.g., phototinitiators such as benzil dimethyl ketal can be used in amounts from about 0.1 to about 5 parts per hundred parts of monomer for ultraviolet light photopolymerized adhesives, and 2,2xe2x80x2-azobis(isobutryonitrile) may be used for solvent polymerization in amounts of from about 0.1 parts to about 2 parts per hundred parts of monomer.
The pressure sensitive adhesives may also include additives such as cross-linking agents, fillers, gases, blowing agents, glass or polymeric microspheres, silica, calcium carbonate fibers, surfactants, and the like. The additives are included in amounts sufficient to effect the desired properties.
The pressure sensitive adhesives may also contain thermosettable resins such as epoxies and urethanes, which can be heat cured after the pressure sensitive adhesive tape has been applied to a surface to form a thermosetting adhesive.
The pressure sensitive adhesives may be prepared by methods that are known in the industry, including solvent polymerization, radiation polymerization by such means as electron beam, gamma radiation, and ultraviolet radiation, emulsion polymerization, and the like. Methods of making pressure sensitive adhesives, for example, are disclosed in U.S. Pat. No. Re 24,906 (Ulrich).
The following non-limiting examples serve to illustrate specific embodiments of the invention.
The release liner film is cut into strips measuring 2.54 cm by 40.64 cm. The pressure sensitive adhesive sample is laminated onto a 5.08 cm wide by 30.48 cm long metal plate. The release liner film is then laminated to the adhesive using 2 passes with 6.8 kilogram roller. The sample is then aged at room temperature (21xc2x0 C.) for 3 hours. The samples are then placed in a peel adhesion tester and the release liner is removed at a 90xc2x0 peel angle at peel rates of 228.6 centimeters/minute (cm/min) and 30.48 cm/min. Results are recorded in grams per 2.54 centimeter width (g/2.54 cm).